With the continuous development of engine theories and technologies, an exhaust gas recirculation (EGR) system has become an important component in a diesel engine. The exhaust gas emitted from a diesel engine generally contains a great amount of nitrogen oxides (NOx), which is a major source for air pollution. With the EGR system, a part of exhaust gas generated by the diesel engine is fed back to cylinders. Since the recirculation exhaust gas is inertia, it will delay the combustion process, decelerate the combustion speed somewhat, and then slow down the pressure formation process in the combustion chamber, thereby effectively reducing the nitrogen oxides. Besides, promotion of the exhaust gas recirculation ratio will reduce the overall exhaust gas flow, thereby reducing the total pollutants output amount in the exhaust gas emission.
Besides EGR, in order to enhance the power performance of the diesel engine and improve combustion, a turbocharging system is also one of important components in modern diesel engines. For example, a common turbocharging system is a variable geometry turbocharger (VGT). The turbocharing system is essentially an air compression system, which increases the intake air amount in the diesel engine cylinders through compressing air. It is driven by the impulsion from the exhaust air emitted from the engine. The pressure is transmitted to an air compressor by devices such as a turbocharger rotating shaft, such that the newly input air is effectively supercharged before entering into the cylinders.
In a diesel engine equipped with both the EGR and the turbocharging system, the coupling characteristic therebetween poses a challenge to the control of air system. In the diesel engine equipped with both the exhaust gas recirculation system EGR and the turbocharging system, for the EGR system, precise control of the EGR ratio and intake air temperature is crucial to improve NOx emission and reduce its impact on particles, power, and cost-effectiveness. In such an engine, the flow of the input exhaust gas of an EGR cooler is controlled by an EGR valve. Both the inlet end of the EGR valve and the turbo inlet of the turbocharger receive the engine exhaust gas emitted from an exhaust pipe. It would be appreciated that besides the variation of the opening degree of the EGR valve itself, the change of the supercharging pressure and exhaust back pressure caused by the charging system would also generate an impact on the EGR flow ratio. On the other hand, the variation of the opening degree of the EGR valve would also generate an impact on the inlet flow ratio of the input supercharger. In other words, the exhaust gas recirculation system and the supercharging system are two mutually dependent and mutually influencing systems, i.e., having a coupling characteristic.
The coupling characteristic of the exhaust gas recirculation system and the supercharging system is always a challenge for air system control of a diesel engine, and a multi-variant control strategy controlling the two is also always a research focus of the air system control strategy of the diesel engine. The prior art has proposed several known control strategies, which are simply summarized below:
(1) an independent control strategy for the exhaust gas recirculation system and the supercharging system, i.e., with the supercharging pressure as the control target, driving the supercharging valve by a PID (proportion-integration-differentiation) control plus transient feed-forward control strategy so as to enable the actual supercharging pressure to reach a target value; with the flow of air as the control target, driving the EGR valve through PID control plus transient feed-forward control policy so as to enable the actual flow of air to reach the target value.
(2) With the intake air flow and the supercharging pressure as control targets, perform local linearization to the average value mode of the air system, design an optimal or robust controller based on the linear model, further extend to the whole working condition scope, thereby obtaining a non-linear control strategy, such as H infinity control, a controller design method based on Lyapunov stability theory, minimum quadratic model optimal state feedback control law, and sliding mode controller, etc.
(3) With the intake air flow and the supercharging pressure as control targets, basing on controller design methods of a non-analytic model, such as, a fuzzy logic control method, a control method according to a neural network, etc.
(4) With the intake air flow and the supercharging pressure as control targets, employing model prediction control methods, i.e., integrating a mathematic model of a controlled object in the controller, predicting a future multi-step system output through the model, building a target function based on the offset between the predicted value and the target value, and minimizing the target function by iteratively evaluating the current control amount.
(5) With the air-fuel ratio and the mass fraction of the exhaust gas in the intake air pipe as the control targets, adopting the air system reduced-rank de-coupling control strategy, i.e., the transmission function matrix of the air system is reduced-ranking in some cases; thus, the two control targets have a certain relationship, such that the original two-dimensional control strategy may be converted into a simpler one-dimensional control strategy.
The major advantages of the independent PID control strategy (1) based on the air flow and supercharging pressure lie in a simpler structure, the capability of implementing a good steady-state control effect, and less experimental workload for parameter calibration. The challenge of the independent closed-ring PIC control lies in that the coupling characteristic of the system per se causes unsatisfactory control effect in its dynamic process, and smoking phenomenon likely appears during the acceleration process. Another drawback of independently working closed-ring control lies in that the EGR working scope is limited, because the EGR valve can only work when the pressure before turbo is higher than the supercharging pressure; therefore, it is only applicable to medium-low load and medium-low rotating speed working condition. Nissan, Toyota, Cummmins, and other companies do not adopt the air flow and supercharging pressure as the target values during the practical use, but adopt a control strategy with the EGR ratio instead of supercharging pressure as the target value.
A common problem between the above methods is EGR flow estimate. Since the EGR flow sensor cannot satisfy the need of actual use by far in terms of precision and reliability, the EGR flow is mainly obtained by estimation. Besides, for temperature and pressure of the exhaust pipe, the EGR pipe throttling coefficient, and cooling efficiency and the like that have an impact on EGR flow, they all require a considerable amount of experiments to obtain a satisfactory estimation effect; therefore, the control system experiment according to this method would be an enormous work. Although the above control strategies can achieve a sound effect in a steady-state control, since the exhaust gas recirculation system and the supercharging system simultaneously act on the intake air pipe and thus have a coupling characteristic, while those control strategies fail to design a transient control strategy for the coupling characteristic, their transient control effect is always unsatisfactory.
There is an apparent contradiction between the precision requirement and the concise requirement of the air system control strategy for the control strategies (2)-(4) with intake air flow and supercharging pressure as control targets. This contradiction is directly caused by the strong coupling and non-linear correlation between the exhaust gas recirculation system and the supercharging system. The independent closed-ring control policy based on the air flow and supercharging pressure, as well as its variations, cannot satisfy the steady-state and transient performance requirements. Various theoretical study outcomes, due to the complexity of control strategies, the requirements of control hardware, and the difficulties in parameter calibration, and other factors, are not suitable for the requirements of an actual control system.
The control strategy (5) with the air-fuel ratio and the mass fraction of the exhaust gas within the intake air pipe as the control targets, due to lack of a mature commercial sensor that directly measures the air-fuel ratio and the mass fraction of the exhaust gas within the intake air pipe during the practical use process, cannot realize a feedback control with the parameters as the control targets. Further, air flow and supercharging pressure can be very easily measured by existing sensors; thus, a feedback control strategy may be built based on the air flow and the supercharging pressure; the air-fuel ration and exhaust gas mass fraction within an intake air pipe, as intermediate variants, may be obtained through an observer. However, the state observer would introduce time delay and error, which are disadvantageous to transient operation control.
In view of the above, the control strategies for an air system of a diesel engine in the prior art cannot simultaneously satisfy well the steady-state and transient working condition performances during the actual operation of the diesel engine or the requirements of the exhaust and the calibration of diesel engine control unit (ECU).
Therefore, it is desirable in this field for an air system control strategy that can satisfy the actual working condition of a diesel engine, is relatively simple and can be easily implemented and calibrated.